Your best bet would be "Bassix" in downtown Vancouver. I don't think they have a website. The address is 217 West Hastings Street. Phone number: (604) 689-7734.
There are other places that will have the odd thing, but that's the best place I know off.
Come to think of it, you may also want to check out "A & B sound" on Seymour street in downtown Vancouver (not far from Bassix). They always have a few things like: Hallucinogen, AP, Cosmosis, and assorted compliations. You have to be prepared to search though the entire electronic section to find them though.
One last thing. If you going to here on Friday, check out "Club 23" on West Cordova (also downtown). Every Friday, they play only psytrance all night there.
Cheers.
P.S. Where are you visiting from?